In general, for a light-emitting diode, the light output depends on the quantum efficiency of the active layer and the light extraction efficiency. The higher the quantum efficiency of the active layer, the higher the light output of the light-emitting diode. Generally, the quantum efficiency of the active layer is increased by improving the quality of epitaxial structure and the structural design of the active layer. In addition, as the light extraction efficiency increases, the light output of the light-emitting diode is enhanced. In order to improve the light extraction efficiency, efforts are made to overcome the significant photon loss resulting from total reflection inside the light-emitting diode after emission from the active layer.
FIG. 1 is a schematic diagram showing the light emission of a conventional light-emitting diode. The light-emitting diode mainly includes a substrate 100, a nucleation layer 102, an N-type semiconductor layer 104, an active layer 106 and a P-type semiconductor layer 108 stacked on the substrate 100 in sequence. FIG. 1 shows that most of the light emitted from the active layer 106 is totally reflected inside the light-emitting diode. After being reflected many times, the light energy is absorbed, thereby greatly reducing the light extraction efficiency of the light-emitting diode.
Typically, roughening a surface of the light-emitting diode-is adopted as a means to overcome the total reflection of the light inside the light-emitting diode. There are many methods to roughen a surface of the light-emitting diode, and most of these methods are performed by back-end processes. However, the back-end processes easily change the electrical properties of the light-emitting diode.
Typically, methods for controlling the epitaxial growth conditions to obtain roughened surfaces are categorized as follows. In one method, the substrate is roughened by an etching technique before an epitaxial process is performed, and then the epitaxial growth is performed on the roughened surface. In applying the method, a micro-rough surface can be formed, so as to enhance the light extraction efficiency of the light-emitting diode. However, the surface roughness formed by the method is not significantly rough, so the light extraction efficiency is not enhanced much. Furthermore, the method is relatively complicated.
In another method, the growth temperature of the P-type cladding layer 208 is reduced during the epitaxial growth process, so as to make a surface of the epitaxial film have a plurality of pits 210, such as shown in FIG. 2. In FIG. 2, the light-emitting diode mainly includes a substrate 200, a nucleation layer 202, an N-type cladding layer 204, an active layer 206 and a P-type cladding layer 208 stacked on the substrate 200 in sequence. A roughened surface formed by the method can decrease the degree of total reflection inside the light-emitting diode of the light emitted from the active layer 206, so that the light extraction efficiency of the light-emitting diode can be enhanced. However, such a roughened surface primarily results from the epitaxial film defects of the pits 210, and most of the pits result from the surface termination of threading dislocations 212. That is, in the epitaxial growth process, the threading dislocations 212 extend from mismatched lattice locations in the substrate 200 along the growth directions of the epitaxial film to the film surface. The end points of such threading dislocations 212 form the pits 210, so that the surface is roughened. Such as shown in FIG. 2, the threading dislocations 212, which form the pits 210 in the epitaxial film, pass through the active layer 206. Accordingly, the threading dislocations 212 become leakage paths, so that the light-emitting diode exhibits excessive leakage current during operation and seriously diminishes the electrical quality of the device.